Motors, Generators and Rotating Equipment
Nameplate data and Code tables
A motor nameplate gives voltage, phase, frequency, full-load amps, service factor, insulation class, design letter (NEMA B is the general-purpose workhorse), efficiency, power factor, and duty. The Code tables give standard full-load currents for sizing conductors and short-circuit protection, while the nameplate value is used for overload protection. That distinction - table current for conductors and branch protection, nameplate current for overloads - is one of the most frequently tested points.
Speed, torque and slip
Synchronous speed = 120 × frequency ÷ number of poles. A four-pole motor on 60 Hz has a synchronous speed of 1800 rpm; loaded, it runs slower, and the difference is slip. Torque is proportional to current at low slip, and horsepower = torque × rpm / 5252 (in imperial units). Reduced-voltage starting reduces starting torque with the square of the voltage reduction, which is why it only suits low-inertia loads.
Generators
Generator output depends on the prime mover (frequency and real power) and excitation (voltage and reactive power). An automatic voltage regulator holds terminal voltage; governors hold speed. Parallel operation requires matching voltage, frequency, phase sequence and phase angle, then sharing kW via the governor and kvar via excitation.
Traps
Confusing service factor with a continuous overload allowance regardless of ventilation; sizing overloads from the Code table instead of the nameplate; ignoring that a VFD-fed motor may need inverter-duty insulation; and forgetting that generator neutral grounding and bonding follow the same rules as transformer secondaries.
Worked example
A 4-pole, 60 Hz motor nameplated at 1750 rpm. Synchronous speed = 120 × 60 / 4 = 1800 rpm. Slip = (1800 − 1750) / 1800 = 2.8%. If it develops 50 lb·ft at that speed, horsepower ≈ 50 × 1750 / 5252 = 16.7 hp, consistent with a 15–20 hp frame depending on service factor.